Automated systems for measuring the concentration of analytes in a sample have been developed using a number of analytical techniques such as chromatography or mass spectrometry. In particular, mass spectrometry is often the technique of choice to achieve sensitivity of parts per billion (ppb) or sub-ppb such as parts per trillion (ppt). For example, co-assigned U.S. Ser. No. 10/004,627 (the '627 application), issued as U.S. Pat. No. 6,974,951, discloses an automated analytical apparatus measuring contaminants which may be present in trace concentrations or constituents which may be present in substantial concentrations using a form of In-Process Mass Spectrometry (IPMS).
In an IPMS technique, a sample of interest is spiked, i.e., has added to it a known amount of the appropriate isotopic species or an internal standard. After the spike and sample have equilibrated, the mixture is ionized using an atmospheric pressure ionization (API) technique such as electrospray and processed in a mass spectrometer to determine a ratio measurement. Depending upon the composition of the spike, the ratio will either be an altered isotopic ratio as used in isotope dilution mass spectrometer (IDMS) or the ratio of an internal standard to the analyte of interest. Unlike the harsh ionization using in inductively coupled mass spectrometry (ICP-MS), the mild ionization provided by the use of API enables the characterization of complex molecules rather than just elemental species. Because a ratio measurement is used, the analysis is immune to drift and other such inaccuracies that plague conventional mass spectrometry analyses.
The IPMS technique represents a dramatic improvement over conventional mass spectrometry methods. Whereas conventional mass spectrometry methods require considerable hands-on intervention from highly-trained analytical chemists, IPMS is completely automated. Because of this automation, IPMS may be used to characterize analytes in fields such as semiconductor clean rooms where the use of mass spectrometry would traditionally be inappropriate. Moreover, this automation may be used to characterize virtually any type of analyte one may be interested in—from elemental species (which may be mono-isotopic) to complex molecular species. However, this automation faces a bottleneck at an electrospray probe used for electrospray ionization. Before a new analysis may be completed, the electrospray probe must be rinsed and then conditioned with the newly-equilibrated spike/sample solution. Having been conditioned, the probe may be used in the characterization of an analyte of interest in the newly-equilibrated spike/sample solution. This delay complicates the analysis of, for example, a copper plating solution in a semiconductor bath in which a user may desire to know the concentrations of a number of plating accelerants, retardants, constituents, and contaminants. To measure each one of these analytes thus entails an appreciable amount of delay because of the associated rinse and conditioning cycles.
Accordingly, there is another need in the art for an improved IPMS apparatus that reduces the delay associated with repetitive rinse and conditioning cycles.